Chemical expansion has significant potential for producing stress, fracture, and strain during high temperature electrochemical device operation. This may have a negative functional impact, leading to cracking or delamination in situ, or it may be turned to a more positive outcome by tuning material performance through mechanical cues including stress and strain. Being able to take advantage of chemical expansion operando while avoiding detrimental mechanical failures fundamentally requires the ability to detect such effects under in situ conditions. This includes characterizing oxides in both equilibrium conditions and dynamic conditions that might represent, for example, gas interruption or redox cycling. Additionally, given the differences known to exist between film and bulk forms of these oxides, thin film-specific characterization methods are required.
There are several ways to characterize chemical expansion. Among these, the most prominent are diffraction-based techniques and dilatometry. Diffraction has many advantages, including a diversity of in situ measurement possibilities and the flexibility to measure films, powders, or pellets and determine orientation-specific information. However, most diffraction instruments require a minimum of ten seconds to achieve a usable signal-to-noise ratio (SNR) threshold, meaning that faster measurements are not possible without the aid of a synchrotron. Furthermore, while diffraction is sensitive to lattice strain or phase changes, it cannot detect new lattice site formation or volume change that is not periodic (e.g., that might be associated with grain boundaries, dislocations, or similar defects).
In contrast, dilatometry is a fairly straightforward type of measuring volume changes caused by a physical or chemical process. For example, a material undergoing chemical expansion may push a rod connected to a strain gauge, causing a change in the strain measured by the strain gauge. Dilatometry is sensitive, on sub-second time scales, to all types of volume change. Unfortunately, dilatometry is better suited to studying bulk samples than to studying thin films.